Heat resistance of a laminate for electronic materials used in flexible printed boards or HDD suspensions has been determined on the premise that a joint temperature at the time when a solder-joint is performed after formation of a circuit or the like is about 240° C. under an environment where a laminate is allowed to stand in a general atmosphere at room temperature and moisture and saturatedly absorbs moisture in air. This is because a melting point of a typical eutectic solder containing lead is 180° C., and a solder joint can be performed at a temperature between 200° C. and 250° C.
However, from the fact that lead is harmful, a lead-free solder has been developed and put into practical use. Because a melting point of a typical lead-free solder is 200–230° C., a joining temperature of 250° C. or higher is required in order to perform a sufficient solder joint. Therefore, a laminate having heat resistance against such a high joining temperature of a lead-free solder is desired.
It is known that a heat resistant polyimide resin is used as an insulating resin in a flexible printed board. Further, it is known as disclosed in JP 08-250860 A that a material containing plural polyimide resin layers is used for an insulating layer. However, the technology disclosed therein was not sufficient in its heat resistance for a lead-free solder joint.
Further, JP 03-164240 A describes that in a laminate used in flexible printed wiring boards or the like, a specific diamine such as tolidine is used as a diamine component constituting a polyimide resin in order to form a laminate that has an excellent adhesiveness between a metal foil and a film and is difficult to curl. However, the laminate described therein is composed of a polyimide single layer, and balance adjustment of various performances required in laminates is difficult in some cases. In addition, heat resistance in a lead-free solder joint is not taken into consideration at all.